Electronic thermostat

ABSTRACT

A thermostat for generating control signals for a furnace, air conditioner, or the like, employs a constant frequency, periodically alternating electrical source and an oscillator incorporating an element having a thermal-sensitive electrical characteristic so the oscillator frequency varies as a function of the ambient temperature. A comparator receives the two outputs and generates a signal representative of the ambient temperature. The stable output is also provided to dividing circuits to generate a signal representative of the time that is used to interrogate a digtal memory storing a desired temperature-time period, such as a week or month. The ambient temperature signal is compared with the desired temperature signal to generate an output control signals. A display provides a read-out of time and temperature and is used along with a keyboard or the like in programming the memory.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an electronic thermostat for generating atemperature dependent control signal employing a digital, programmablememory time-temperature.

2. Prior Art

Thermostats which act to generate an electrical control signal for afurnace, air conditioning unit, or the like, typically employ a bimetalelement which assumes a mechanical position dependent upon the ambienttemperature. The desired temperature may be set into the device bymoving an electrical contact with respect to the bimetal so that anoutput signal is provided when the ambient temperature reaches the setpoint. To allow this set point to be automatically varied over a 24-hourcycle, thermostats are available which incorporate clocks whichmechanically adjust the thermostat set points at different times. Whenused in a residence to control a furnace this enables the temperature tobe lowered during the night and at other times when the occupants arenot normally home. These program devices typically allow a 24-hourperiod to be divided into two blocks of controllable length, and allow adifferent temperature setting for each block.

In view of the cost and reliability advantages of electronic circuitry,temperature sensing devices which employ an electronic element havingelectrical characteristics which vary as a function of temperature, suchas a thermistor, are often employed in industrial devices. When used asthermostats these devices are relatively complicated since they requirecircuitry for generating an analog voltage proportional to the set pointand a comparator for receiving this analog voltage and the output of thetemperature sensitive device. While it would be possible to add a24-hour programming clock to this form of analog thermostat, theprovision of a clock mechanism would probably obviate the advantages ofthe electronic temperature sensing elements over mechanical elements.

Independent of these developments in thermostats great cost reductionsin digital electrical circuitry have taken place in recent years throughthe development of integrated circuits incorporating large numbers ofdigital devices on a single semi-conductor chip. Once the initialtooling costs for a specific integrated circuit have been amortizedlarge quantities of complicated circuits may be produced at very lowcosts. These advances have been possible devices such as low costelectronic calculators and digital watches.

The present invention is broadly directed toward an electronicthermostat employing digital circuitry so that it may be mass-producedin the form of a low cost integrated circuit. Thermostats formed inaccordance with the invention enjoy advantages over conventionalthermostats in both cost and reliability and provide a variety ofauxiliary functions which would be extremely expensive to implement inconventional mechanical or analog electronic thermostats.

SUMMARY OF THE INVENTION

The thermostat of the present invention broadly employs an oscillatorincorporating an electronic element with properties which vary as afunction of the ambient temperature so that the frequency of theoscillator varies with the temperature. The device also employs acircuit having a periodically varying output of constant frequency. Thismay take the form of a highly stable oscillator or the periodicallyvarying signal may be derived from alternating current electric powerlines.

The constant frequency signal is employed in two ways: first, itgenerates a time base which allows a signal which is a function oftemperature to be derived from the output of the temperature sensitiveoscillator, and second, it provides the input to a set of digitaldividing counters which generate time signals. The time signals mayinclude the year, month and day of the month as well as hour, minute andsecond. These time signals are used to interrogate a digital,programmable memory which stores a time-temperature sequence; i.e., arecord of the desired temperature at each time within a repetitiveperiod such as a day, week, month or year. The desired temperaturesignal derived from the memory by the time signal is then compared withthe actual temperature signal and their differences are used to generatea control signal for the furnace, air conditioner or the like which maybe either proportional or an on-off signal.

The occurrence of the output signal from the thermostat may be adjustedto compensate for the time required for the controlled device to affectthe temperature at the thermostat; for example, if the desiredtemperature is 70° F. and the temperature sensed by the thermostat isdropping, the control signal to the furnace may go on at 70.5° F. This"compensation signal" may be adjustable and may be derived by the systemas a function of the rate of change of temperature or on the basis ofexternally sensed factors such as temperature outside of the building.

The preferred embodiment of the system incorporates a dedcimal displaywhich may display actual temperature, set point temperature, and/ortime. These may be displayed continuously or on a sequential basis. Thedisplay may, for example, alternately flash the date, the time and theactual temperature. If an external temperature probe is used with thesystem the outside temperature could be similarly displayed.

The display is also used to program the time-temperature schedule in thememory through use of manual controls associated with the thermostat.The programming device takes the form of a group of low-costpush-buttons of the type used with digital watches. One button switchesthe device between an operating mode and a set mode; another buttonselects the register to be incremented, i.e., present date, presenttime, future date, future time, temperature; a third button may be usedto actually step these functions, which are then displayed, throughvarious settings. The buttons or other manual control allow a particularstart time and date to be inserted into the memory and displayed, andthen allow a temperature for the period beginning at that time to beentered and displayed. Next the end time of one period and the beginningtime of the next period and the desired temperature for that subsequentperiod are entered. This process is continued until the entire period ofthe control is covered. For example, a week may constitute a convenientcycle period. A relatively high temperature may be programmed into thememory for those periods during the week when the house is occupied andthe occupants are normally awake, and lower temperatures may beprogrammed for the sleeping hours and when the house is unoccupied. Theprogram may be designed to begin to warm up the house shortly before theoccupants normally wake or before they return home.

The ability to program a full time cycle of temperatures lasting overlong periods of time results in important energy savings throughdecreased use of fuel or power during period when heating and coolingmay be decreased without discomfort to occupants or inconvenience tocommercial or industrial users.

Means may be provided for allowing a normal program to be retained butto be overriden by a programmed temperature for a specified period oftime. For example, if the occupants are going to leave the house for theweekend they may program a lower temperature during that period with awarm-up cycle beginning shortly before they return. After thisprovisional period has been completed the control resumes its normalcycle.

Electrical power for the thermostat is preferably derived from analternating current power source such as a 24 volt secondary coil of atransformer of the type used to power the fuel line solenoid of afurnace. The switching output of the thermostat is preferably connectedin series with the load to be controlled across the power lines. Whenthe switching output is open so that the controlled device is turnedoff, electric power for the control is available across the terminals ofthe switch. When the switch output is closed, energizing the calleddevice, alternating current power for the thermostat may be developedacross a small resistance connected in series with the controlled deviceand the power lines.

While it would be possible to derive an analog electrical signal havingan amplitude which varies as a function of temperature from the variablefrequency output of the temperature dependent oscillator without the useof some form of time base, the use of analog circuitry would tend toincrease the complexity and expense of the thermostat over an alldigital version. Derivation of a digital signal having a valuerepresentative of temperature inherently requires use of some form oftime base with the variable frequency oscillator.

The preferred embodiment of the invention employs integratedsemi-conductor circuits to implement all of the digital functionsincluding the oscillator, time base, memory and the comparators. Thiscircuitry may take the form of one or more integrated circuit chips withinterconnections to the display, the temperature sensing element, aprogramming device and the output switch. The output switch may take theform of a solid state switch or hard contact. If a solid state switch isemployed it may or may not be formed as part of the integrated circuitdepending upon various economic and technical factors. The logiccircuitry of the thermostat is preferably implemented with a suitablyprogrammed microprocessor.

The cost of the present thermostat is comparable to that of anelectronic calculator or a digital watch and is thus lower in cost thanbimetal therostats that it replaces despite the large number ofadditional functions that it performs.

Other objectives, advantages and applications of the present inventionwill be made apparent by the following:

FIG. 1 is a block diagram of an electronic thermostat representing apreferred embodiment of the invention connected to a load so as tocontrol the application of electrical power to the load;

FIG. 2 is a more detailed partially schematic, partially block diagramof certain sections of the thermostat of FIG. 1; and

FIG. 3 is a block diagram of an alternative arrangement for portions ofthe thermostat of FIG. 1.

Referring to FIG. 1, the preferred embodiment of the thermostat employsa stable periodic source 10. The source may constitute a stableoscillator, possibly employing a crystal, or it may take the form of acircuit which provides a periodically switching output using thealternating current power mains as a timing source, as will besubsequently described. The primary characteristic of the stable source10 is that it is insensitive to the ambient temperature at thethermostat, at least over the normal operating range of the device. Itsoutput constitutes a bi-valued electrical signal which periodicallychanges state. If the device is of the type that produces an analogvarying output, such as a sinusoidal oscillator, appropriate waveshaping circuitry, such as a zero crossing detector, may be used toconvert the signal into a bi-valued signal.

The output of the stable source 10 is provided to a dividing chain 12which generates real time signals, i.e., output signals that changestate each second, minute, hour, and day. In alternative embodiments ofthe invention the divider chains could provide additional signals whichchange state with the month and year, but the preferred embodiment ofthe invention operates on a one week cycle and accordingly no month oryear signals are required. The dividing chain 12 may be of the typeemployed in digital clocks or watches.

The periodic source 10 also provides an output to a temperature detectorcircuit 14. Alternatively, the time base signal to the temperature couldbe derived from some stage of the divider chain 12.

The other input to the temperature detector 14 is from a variablefrequency oscillator 18 which employs a thermal sensitive element 20.The element 20 is physically mounted on the thermostat so as to besubjected to ambient temperature. It has some electrical property thatvaries as a function of ambient temperature at least over the normalrange of operation of the thermostat. For example, the temperaturesensitive element 20 could constitute a thermistor connected across anelectric power supply to provide a voltage to the oscillator 18 as afunction of the ambient temperature on the thermostat. In that case theoscillator would be of the voltage controlled type and would provide aperiodically varying output having a frequency dependent upon theambient temperature. Alternatively, the element 20 could be connected inthe tank circuit of the oscillator 18. Taken together, the oscillator 18and the temperature sensitive element 20 constitute a temperaturedependent frequency source.

The detector 14 uses the time base derived from the stable periodicsource 10 to generate a temperature dependent digital signal from theoutputs of the temperature dependent oscillator 18. The detector circuitmay take a variety of forms, some of which will be subsequentlydescribed in detail. For example, the detector could employ counts toreceive its two inputs and periodically compare their states, as bysubtracting them, to generate a temperature dependent signal.Alternatively, one input could be used to increment a bi-directionalcounter and the other input used to decrement it. The contents of thiscounter could be examined at regular intervals and would constitute atemperature dependent signal.

The outputs of the time generating divider chain 12 are employed tointerrogate a time-temperature programmed memory 22 to obtain a read-outfrom the memory of a desired temperature, for that time, which is storedin the memory. In the preferred embodiment of the invention the controloperates on a one week cycle and the memory 22 stores desiredtemperature values for all times within a one week cycle. This desiredtemperature is provided to a comparator 24 which also receives theactual temperature from the detector 14. The comparator provides anoutput signal having a characteristic dependent upon these two inputs.The output signal may be a two-state signal having one value when theactual temperature is greater than the desired temperature, and theopposite value when the actual temperature is less than the desiredvalue. Alternatively, the comparator 24 could generate a proportionaloutput signal representative of the difference between the actual andthe desired temperatures.

The comparator output could also be modified to consider such factors asthe lag between the time the controlled device is energized and the timethe temperature change actually reaches the thermostat; i.e., thecomparator output could be adjusted to provide a switching output whenthe actual temperature is moving in the direction of the desiredtemperature but has not yet reached it. For example, if the controlleddevice 26 is a furnace, an output could be provided from the comparatorwhen the actual temperature is decreasing and reaches a predeterminedvalue above the temperature set point. Toward this end, the comparator24 may have an input from an external temperature sensor 28 so that thedegree of offset may be made a function of the difference between theactual temperature and the external temperature since the actualtemperature will change at a rate dependent upon this difference. Inalternative embodiments the comparator could employ circuitry tocalculate the rate of change of actual temperature to adjust the leadfactor.

The output of the comparator 24 controls an output switch 30. Typically,the switch will be connected in series with an electrical power source32 and the controlled load 26 so that the controlled device is energizedwhen the output switch is closed. The output switch may employmake-and-break contacts, which may either be hard contacts or a solidstate switch. When the comparator output is proportional the duty cycleof the output switch may be controlled to adjust the flow of power tothe controlled device.

Power for the circuit is preferably obtained from the source 32 bytapping across the switch contacts when they are open, or across aseries resistor 31 in the output circuit when the switch contacts areclosed.

The program stored in the memory 22 is loaded in and modified by asuitable program source 34. In the preferred embodiment of the inventionthe program source takes the form of a keyboard containing numbers andcommands but alternatively the program source could be push buttons ofthe type that are used in digital watches, thumb wheels, or the like.

A display device 36, preferably taking the form of a liquid crystaldisplay, receives a binary signal from a display selector 38. Inalternative embodiments of the invention the display device could takethe form of light emitting diodes, a gas plasma display panel or thelike. The display selector receives an actual temperature signal fromthe digital detector 14; a time signal from the divider chain 12; thedesired temperature signal at the instant from the memory 22 and ageneral output from the memory 22 on line 40. During normal operation,the selector 38 provides signals to the display 36 which causes it tosequentially display the time, the actual temperature, and the desiredtemperature. Alternatively, other information such as the externaltemperature as generated by the sensor 28 could be displayed.

When the program source 34 is used to load or modify the memory 22, thedisplay selector is controlled by the signal from the source 34 whichcauses it to display the information being provided to the memory fromthe source, as generated on line 40 from the memory. As particularinformation is entered into the source 34 by the programmer, it is shownon the display.

Certain of the sections of the preferred embodiment of the thermostatare illustrated in more detailed block form in FIG. 2.

The stable periodic source 10 preferably takes the form of a 60 cyclesource 50 feeding a zero-crossing detector 52. The zero-crossingdetector provides an output pulse each time the 60 cycle source passesthrough zero. A flip-flop 54 converts the output of the zero crossingdetector into a square wave form.

The outputs of the zero crossing detector are provided to the dividerchain 12 which has outputs for minutes, hour, am/pm, and day.

The output of the flip-flop 54 is also provided to a counter 56contained within the temperature detector 14. Similarly, the output ofthe variable frequency temperature dependent oscillator 18 is providedto another counter 58. The outputs of these two counters provided to asubtractor unit 60 and subtraction is initiated at regular intervals bya signal on the minute output of the divider chain. This signal alsoclears the counters 56 and 58. Accordingly, once each minute asubtractor generates a signal equal to the difference between thefrequencies of the stable and temperature source.

The time-temperature program memory 22 includes a random access memorywhich may be represented as containing four separate sections, aday/schedule matrix 62 and three schedule sections 64, 66 and 68. Theschedule sections are each loaded with a complete 24 hourtime-temperature cycle. A typical schedule might be: 12 am-7 am-65°; 7am-9 am-70°; 9 am-5:30 pm-55°; 5:30 pm-11:30 pm-70°; 11:30 pm-12 am-65°.A typical schedule for a day when the home is to be unoccupied might be12 am--12 am-55°.

Three such schedules are loaded into the memory sections 64, 66 and 68.In alternative embodiments, additional schedule sections could beprovided.

The day/schedule matrix is loaded with information as to which scheduleis to be used on a particular day. For example, Monday-Friday might allinvolve schedule A; Saturday schedule B; and Sunday schedule C.

One particular schedule from the matrix 62 is selected by the "day"output of the divider chain 12. This causes matrix 62 to energize one ofthree gates 70a, b or c to connect one of the three schedules, 64, 66 or68, to a temperature selector 72. The selector also receives the"minute", "hour" and "am/pm" outputs of the divider chain 12. It usesthis time signal to interrogate the selected schedule to provide anoutput to the comparator 24 which represents the desired temperature atthat time.

Alternate forms of program arrangement could be used in otherembodiments of the invention. For example, the thermostat could beprogrammed on simply a daily basis or a monthly basis.

Certain alternative sections for the thermostat of FIG. 1 areillustrated in FIG. 3. These sections allow the thermostat to control atemperature system comprising an air cooler or conditioner 80 and an airheater 82 such as a furnace or the like. The alternative embodiment ofthe thermostat also provides programmable units for adjusting thetemperature generating system to compensate for inaccuracies of thetemperature dependent oscillator both as to base frequency and rate ofchange of frequency as a function of temperature.

The system employs a thermal sensitive oscillator 84. The frequency ofthis oscillator may be expressed as:

f=kt+f_(o) where:

f=observed frequency of oscillator;

t=temperature;

f_(o) =the fundamental frequency of the oscillator at a giventemperature; and k is a constant.

Because of the manufacturing variances it may be necessary to compensatethe oscillator by effectively adjusting the fundamental frequency andthe constant. In order to achieve this the circuit of FIG. 3 providesthe output of the oscillator 84 to a counter 86 and periodically clearsthe counter with a signal derived from a stable periodic source 88. Thecount contained in counter 86 each time it is cleared is provided to asubtractor 90 which also receives a digital number from a circuit 92which stores a base temperature adjustment. The contents of the register92 may be adjusted from a program source 94 either at the factory or inuse.

The output of the subtractor 90 is fed through a divider 96 whichdivides the output of the subtractor by a signal from a register 98containing a number which affects the rate of change of frequency as afunction of temperaure adjustment. The contents of the register 98 arelikewise adjusted from the program source 84.

The output of the divider 96 provides an accurate measurement of actualtemperature. Adjustments in the contents of the register 92 effectivelyadjust the fundamental frequency of the oscillator and adjustments inthe register 98 modify the rate of change of the output signal todivider 96 as a function of temperature.

The output of the divider 96 is provided to a pair of comparators 100and 102. The comparator 100 also receives the content of an adder 104which adds a lead factor, stored in register 106, to the desiredtemperature signal as derived from memory 22 or a similar source. Thislead factor may be 3° for example so that when the desired temperatureis 70° the output of the adder 104 is a digital number representative of73°. Similarly, the lead factor stored in register 106 is subtractedfrom the desired temperature signal in subtractor 108 which provides theremainder to another comparator 110. When the actual temperature is 70°and the lead factor is 3°, the subtractor 108 will output a digitalsignal representative of 67°.

When the actual temperature exceeds the output of the adder 104 a signalis provided to an output switch 112 causing the switch to close. Theswitch 112 actuates the air conditioner 80. Similarly, when the actualtemperature signal from the divider 96 is less than the output of thesubtractor 110, a signal is provided to an output switch 114 whichcontrols the heater 92.

This system thus provides a dead zone equal to twice the value set inthe register 106. When the temperature exceeds the top end of that deadzone the air condition is energized and when the actual temperaturefalls below the bottom of the dead zone the heater is energized.

In alternative embodiments of the invention separate lead factors couldbe provided for the air conditioner and the heater. The lead factor maybe preset or programmable as from the program source 94.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows: .[.1. A thermostat forcontrolling the application of electrical power to a temperaturemodifying load, comprising: means for generating an electrical signalhaving a characteristic which varies as a function of the ambienttemperature on the thermostat; a clock operative to generate digitalelectrical signals having values representative of real time; aprogrammable, digital memory; means for loading said memory with aplurality of separate programs each comprising digital signalsrepresentating desired temperature at particular times over a repetitivetime cycle; means for selecting one of said plurality of programs,circuitry for applying the output of the clock to the memory to generatea digital electric signal representative of the desired temperature at aparticular time as stored in said selected program; means for receivingthe electrical signal having a characteristic which is a function ofambient temperature and the output of the memory and for generating acontrol signal for said temperature modifying devices as a function oftheir difference..]. .[.2. The thermostat of claim 1 including a displaydevice and means for connecting the display device to said electricalsignal having a characteristic which varies as a function of ambienttemperature and to the output of the clock..]. .[.3. The thermostat ofclaim 1 including manually operable means for programming saidmemory..]. .[.4. The thermostat of claim 1 wherein said clock comprisesa digital divider chain driven by a periodically varying electricalsignal having a frequency independent of the ambient temperature of thedevice..]. .[.5. The thermostat of claim 4 wherein said means forgenerating an electrical signal having a characteristic which varies asa function of the ambient temperature on the thermostat includes meansfor receiving said periodically varying signal and said electricalsignal having a characteristic which varies as a function of the ambienttemperature on the thermostat..]. .[.6. The thermostat of claim 4wherein said electrical signal having a characteristic which varies as afunction of the ambient temperature on the thermostat comprises anoscillator having a temperature dependent element in its tankcircuit..]. .[.7. The thermostat of claim 1 wherein the electricalsignal having a characteristic which varies as a function of the ambienttemperature in the thermostat is digital..]. .[.8. The thermostat ofclaim 7 wherein the thermostat is formed as an integrated semi-conductorcircuit..]. .[.9. A thermostat for generating control signals for atemperature modifying device, comprising: means for generating a digitalelectrical signal representative of the ambient temperature on thethermostat; means for generating a digital electrical signalrepresentative of the time within a repetitive time cycle; aprogrammable digital memory; means for loading said memory with aplurality of separate programs each comprising digital signalsrepresentating desired temperature at particular times over a repetitivetime cycle; means for selecting one of said plurality of programs;circuit means for interrogating said memory with said digital timesignal to generate a digital signal representative of the desiredtemperature at the existing time as stored in said selected program; anda comparator operative to receive the digital signal representative ofthe ambient temperature on the thermostat and the digital signalrepresentative of the desired temperature, and to provide a controlsignal to said temperature modifying device..]. .[.10. The thermostat ofclaim 9 wherein said means for generating a digital electrical signalrepresentative of the time within a repetitive time cycle includes meansfor generating a constant frequency periodically varying electricalsignal, and said means for generating a digital electrical signalrepresentative of the ambient temperature on the thermostat includes atemperature sensitive oscillator and means for receiving the output ofthe oscillator and the constant frequency signal..]. .[.11. Thethermostat of claim 9 wherein said means for generating a digital timesignal comprises an oscillator which has a constant frequency output atambient temperatures over the range of the thermostat and a dividerchain operative to receive the output of the oscillator..]. .[.12. Thethermostat of claim 9 wherein said means for loading said memory with aprogram of desired temperatures for different times over a repetitivetime cycle includes a manually operable keyboard..]. .[.13. Thethermostat of claim 9 including digital means for modifying the outputof the memory to generate a signal which is provided to the comparatorin order to offset the temperature occurrence of the control signal withrespect to the ambient temperature..]. .[.14. The thermostat of claim 1including a source of a constant frequency, periodically alternatingelectrical signal forming part of both said means for generating anelectrical signal having a characteristic which varies as a function ofthe ambient temperature on the thermostat and said clock..]. .[.15. Athermostat for generating control signals for a temperature modifyingdevice comprising: digital time generating means..]. .Iadd.18. Athermostat for controlling the application of electrical power to aspecified electrically controlled temperature modifying devicecomprising:a source of a periodically alternating electrical signalwhose frequency is constant at ambient temperatures over the range ofthe thermostat; a source of a periodically alternating electrical signalwhose frequency varies as a function of the ambient temperature on thethermostat; circuit means, responsive to the respective frequencies ofsaid time-base electrical signal and said temperature-sensitiveelectrical signal, for generating a digital electrical signalrepresenting the ambient temperature on the thermostat; a clockutilizing an output from said source of a constant frequency electricalsignal to generate digital electrical signals having valuesrepresentative of real time; a programmable, digital memory; means forloading said memory with a plurality of separate programs foralternative use in controlling the specified temperature modifyingdevice, each said program comprising digital signals representingdesired temperatures at particular times over a twenty-four hour period;means for selecting one of said plurality of said programs; circuitryfor applying the output of the clock to the memory to generate a digitalelectric signal representative of the desired temperature at aparticular time as stored in said selected program; and means forreceiving the electrical signal and the output of the memory and forgenerating a control signal for said specified temperature modifyingdevice as a function of the difference between the ambient temperatureand the desired temperature; whereby, through the use of said means forselecting one of said programs the ambient temperature at the thermostatmay be controlled in accordance with different days of theweek..Iaddend. .Iadd.19. A thermostat for controlling the application ofelectrical power to a specified electrically controlled temperaturemodifying device comprising:a source of a periodically alternatingelectrical signal whose frequency is constant at ambient temperaturesover the range of the thermostat; a source of a periodically alternatingelectrical signal whose frequency varies as a function of the ambienttemperature on the thermostat; means for comparing the frequency of saidconstant frequency signal with the frequency of said varying frequencysignal to generate a signal representing the ambient temperature on thethermostat; a clock utilizing an output from said source of a constantfrequency signal to generate digital electrical signals having valuesrepresentative of real time; a programmable, digital memory; means forloading said memory with a plurality of separate programs foralternative use in controlling the specified temperature modifyingdevice, each said program comprising digital signals representingdesired temperatures at particular times over a twenty-four hour period;means for selecting one of said plurality of programs; circuitry forapplying the output of the clock to the memory to generate a digitalelectric signal representative of the desired temperature at aparticular time as stored in said selected program; and means forreceiving the signal having a characteristic which is a function ofambient temperature and the output of the memory and for generating acontrol signal for said specified temperature modifying device as afunction of the difference between the ambient temperature and thedesired temperature, whereby, through use of said means for selectingone of said programs the ambient temperature at the thermostat may becontrolled in accordance with different days of the week..Iaddend..Iadd.20. A thermostat for controlling the application of electricalpower to a specified electrically controlled temperature modifyingdevice comprising:a source of a periodically alternating electricalsignal whose frequency is constant at ambient temperatures over therange of the thermostat; a source of a periodically alternatingelectrical signal whose frequency varies as a function of the ambienttemperature on the thermostat, said variation being measured from a basefrequency at a base temperature; a counter for periodically generating adigital count representative of the difference between the current valueof said temperature-variable frequency and the base frequency; means forsubtracting a digital base-temperature adjustment number from each saidcount and to derive from the result a digital signal representative ofthe ambient temperature on the thermostat; a clock utilizing an outputfrom said source of a constant frequency signal to generate digitalelectrical signals having values representative of real time; aprogrammable, digital memory; means for loading said memory with aplurality of separate programs for alternative use in controlling thespecified temperature modifying device, each said program comprisingdigital signals representing desired temperatures at particular timesover a twenty-four hour period; means for selecting one of saidplurality of programs; circuitry for applying the output of the clock tothe memory to generate a digital electric signal representative of thedesired temperature at a particular time as stored in said selectedprogram; and means for receiving the electrical signal having acharacteristic which is a function of ambient temperature and the outputof the memory and for generating a control signal to the specifiedtemperature modifying device as a function of the difference between theambient temperature and the desired temperature; whereby, through use ofsaid means for selecting one of said programs the ambient temperature atthe thermostat may be controlled in accordance with different days ofthe week..Iaddend. .Iadd.21. A thermostat for controlling theapplication of electrical power to a specified electrically controlledtemperature modifying device comprising:a source of a periodicallyalternating electrical signal whose frequency is constant at ambienttemperatures over the range of the thermostat; a source of aperiodically alternating electrical signal whose frequency varies as afunction of the ambient temperature on the thermostat, said variationbeing measured from a base frequency at a base temperature; a counterfor periodically generating a digital count representative of thedifference between the circuit value of said temperature-variablefrequency and the base frequency; means for dividing said count by adigital temperature-slope adjustment number and to derive from theresult a digital signal representative of the ambient temperature on thethermostat; a clock utilizing an output from said source of a constantfrequency signal to generate digital electrical signals having valuesrepresentative of real time; a programmable, digital memory; means forloading said memory with a plurality of separate programs foralternative use in controlling the specified temperature modifyingdevice, each said program comprising digital signals representingdesired temperatures at particular times over a twenty-four hour period;means for selecting one of said plurality of programs; circuitry forapplying the output of the clock to the memory to generate a digitalelectric signal representative of the desired temperature at aparticualr time as stored in said selected program; and means forreceiving the electrical signal having a characteristic which is afunction of ambient temperature and the output of the memory and forgenerating a control signal to the specified temperature modifyingdevice as a function of the difference between the ambient temperatureand the desired temperature; whereby, through use of said means forselecting one of said programs the ambient temperature at the thermostatmay be controlled in accordance with different days of theweek..Iaddend. .Iadd.22. A thermostat for controlling the application ofelectrical power to a specified electrically controlled temperaturemodifying device comprising:a source of a periodically alternatingelectrical signal whose frequency is constant at ambient temperaturesover the range of the thermostat; a source of a periodically alternatingelectrical signal whose frequency varies as a function of the ambienttemperature on the thermostat, said variation being measured from a basefrequency at a base temperature; a counter for periodically generating adigital count representative of the difference between the current valueof said temperature-variable frequency and the base frequency; means forsubtracting a digital base-temperature adjustment number from each saidcount to generat an intermediate result; means for dividing saidintermediate result by a digital temperature-slope adjustment number togenerate a final result and to derive from the final result a digitalsignal representative of the ambient temperature on the thermostat; aclock utilizing an output from said source of a constant frequencysignal to generate digital electrical signals having valuesrepresentative of real time; a programmable, digital memory; means forloading said memory with a plurality of separate programs foralternative use in controlling the specified temperature modifyingdevice, each said program comprising digital signals representingdesired temperatures at particular times over a twenty-four hour period;means for selecting one of said plurality of programs; circuitry forapplying the output of the clock to the memory to generate a digitalelectric signal representative of the desired temperature at aparticular time as stored in said selected program; and means forreceiving the electrical signal having a characteristic which is afunction of ambient temperature and the output of the memory and forgenerating a control signal to the specified temperature modifyingdevice as a function of the difference between the ambient temperatureand the desired temperature; whereby, through use of said means forselecting one of said programs the ambient temperature at the thermostatmay be controlled in accordance with different days of theweek..Iaddend. .Iadd.23. A thermostat for controlling the application ofelectrical power to a specified electrically controlled temperaturemodifying device comprising: means for generating an electrical signalhaving a characteristic which varies as a function of the ambienttemperature on the thermostat; pi pi a clock operative to generatedigital electrical signals having values representative of real time; asource of a constant frequency, periodically alternating electricalsignal forming part of both said means for generating an electricalsignal having a characteristic which varies as a function of the ambienttemperature on the thermostat and said clock; a programmable, digitalmemory; means for loading said memory with a plurality of separateprograms for alternative use in controlling the specified temperaturemodifying device, each comprising digital signals representing desiredtemperatures at particular times over a twenty-four hour period; meansfor selecting one of said plurality of programs; circuitry for applyingthe output of the clock to the memory to generate a digital electricsignal representative of the desired temperature at a particular time asstored in said selected program; and means for receiving the electricalsignal having a characteristic which is a function of ambienttemperature and the output of the memory and for generating a controlsignal to the specified temperature modifying device as a function oftheir difference, whereby, through use of said means for selecting oneof said programs the ambient temperatures at the thermostat may becontrolled in accordance with different days of the week..Iaddend.